Encoding systems for encoding an audio signal to compress an amount of audio signal data having a plurality of channels have been developed recently. In particular, there has been proposed an encoding system that improves compression efficiency by encoding signals generated by downmixing signals from a plurality of channels. The parametric stereo system and the MPEG Surround system standardized by the Moving Picture Experts Group (MPEG) are known as such types of encoding systems.
In these encoding systems, spatial information and main signals representing the main components of the original channel signals are generated by downmixing the plurality of channel signals and then encoded. Residual signals representing components that are orthogonal to the main signals are further calculated in these systems and the residual signals are also encoded.
Encoders preferably include encoded data of the residual signals along with encoded data of the main signals in encoded audio signals in order to suppress deterioration of sound quality. On the other hand, to further improve compression efficiency, it is preferable that the residual signals are not included in the encoded audio signals. To satisfy such contradictory requirements, Japanese National Publication of International Patent Application No. 2008-519307, for example, proposes a technique to attenuate time regions or signal portions having little perceptual relation within the residual signals. Thus, attenuation of the residual signals increases as the ratio of the residual signal power with respect to the main signal power decreases. Alternatively, only residual signals having frequencies lower than a specific frequency are selected.